One type of internal combustion engine which includes a combustion chamber that generally has a high thermal efficiency and a low fuel consumption is a diesel engine of the direct injection type as disclosed, for example, in Japanese Utility Model Laid-Open No. 59-81775 and shown in FIGS. 14 and 15 of the drawings hereof.
The combustion chamber arrangement mentioned above includes a fuel injection nozzle b and a plurality of glow plugs m secured to a cylinder head a such that heating portions m.sub.1 of the glow plugs m are accommodated in a combustion chamber f at the top i of a piston h when the piston h is positioned at the top dead center thereof as shown in FIG. 14.
The fuel injection nozzle b includes, as shown in FIG. 15, a nozzle body j forming an outer block and a needle valve c mounted for reciprocating movement within the nozzle body j. The nozzle body j has formed therein auxiliary nozzle holes e for atomizing and injecting fuel therethrough when the needle valve c is lifted an amount smaller than a predetermined lift value, and main nozzle holes e form atomizing and injecting fuel therethrough when the neddle valve c is lifted an amount greater than the predetermined lift value. The auxiliary nozzle holes d are formed in a direction such that atomized fuel may be injected therethrough toward the heating portions m.sub.1 of the glow plugs m when the piston h is positioned at its top dead center. Meanwhile, the main nozzle holes are formed in a direction such that atomized fuel cannot be injected therethrough toward the heating portions m.sub.1 of the glow plugs m when the piston h is positioned at its top dead center. It should also be noted that in the arrangement described above, a plurality of the glow plugs m are provided, and the main nozzle holes e and the auxiliary nozzle holes d are formed individually equal in number to the glow plugs m.
In the arrangement described above, fuel (light oil) is first atomized by and injected through the auxiliary nozzle holes and is ignited and burnt by the glow plugs, and then fuel (light oil) is atomized by and injected through the main nozzle holes and is ignited and burnt by propagation of flame thereto. Thus, the arrangement described above was intended to attain high combustion performance in terms of fuel costs, high output power and a reduction in ignition lag.
However, fuel which can be used in the diesel engine of the mentioned type is limited to light oil, and fuel having a low cetane number and a high volatility such as alcohol or gasoline cannot be used in the engine. This is because injected fuel is evaporated instantaneously due to the fact that the amount of fuel injected through the auxiliary nozzle holes upon low load running of the engine is small relative to the volume of the combustion chamber from the viewpoint of the construction of the fuel injection nozzle and that the volatility of the fuel is high.
Accordingly, under low engine load conditions, the concentration of air fuel mixture within the combustion chamber becomes excessively lean. Consequently, it is difficult to produce rich mixture around the glow plugs.